1. Field
The present specification generally relates to methods for reducing speckle in images and, more specifically, to methods for reducing speckle in images projected with laser light sources.
2. Technical Background
Laser speckle causes undesirable effects associated with the use of coherent illumination sources in general and laser-based projection systems in particular. Laser speckle is an undesirable random intensity variation perceived by the eye or other detection system. Speckle may result when a coherent light source is used to illuminate a rough surface such as a screen, wall, or any other object that produces a diffused reflection or transmission. In particular, a multitude of small areas of the screen or other reflecting objects scatter light into a multitude of reflected beams with different points of origination and different propagation directions. At an observation point, such as the eyes of an observer or an optical detector such as the sensor of a camera, these reflected beams interfere constructively to form a bright spot or destructively to form a dark spot, thereby producing a random granular intensity pattern known as speckle.
The standard quantitative measure of the amount of speckle visible is speckle contrast. Speckle contrast is defined as the ratio between the standard deviation of intensity and the mean intensity detected by the eye or another optical detector. When the speckle contrast is 100% the speckle is considered to be “fully developed” which yields the lowest image quality to an observer and/or an optical detector. The human eye can detect speckle at contrast levels as low as 5% or even 1% depending on a number of variables including the wavelength of the projected light and brightness of the illumination as well as variations in perception amongst different individuals.
There is significant interest in image projectors based on coherent laser sources instead of incoherent lamps or LEDs. This is because of the potential compactness, power efficiency, brightness, color saturation and other properties afforded by laser sources. Because of the ability of the human eye to detect speckle in a projected image, even modest amounts of speckle may be distracting to an observer and detract from the perceived quality of a projected image.
Accordingly, a need exists for alternative methods for operating scanning laser projectors to reduce the amount of speckle in images projected with the scanning laser projectors.